Save As Saves Time
You don't have to be a designer to take advantage of this concept.
Imagine you're opening a document in Word, Google Slides, Keynote, I don't care what. Let me guess that you might start with a new blank document. Or you might use a built-in template and modify it to your needs. But ask yourself if you have ever created something similar that is already in a format you like or need. Probably yes. So, let me introduce you to the timesaving concept of Save As. It may look different in different platforms. It might be Save a Copy or something like that. Same thing.
Designers use this all the time. The idea is that you take something where you like the format, make a direct copy so you keep the original intact, and then modify that copy as needed. There is zero penalty for number of copies made. If I have an ad that needs to be revised with a new headline, I don't start from a blank page. I Save As. If I have a slide deck that needs to use the same format and colors as a previous one, I don't start from scratch. I Save As. If I have a layout that I like, but want to try something wacky, I Save As.
You can think of this as either Form or Content first. You'll merge the two eventually, but you have a choice where to start.
Form First:
If you need to adhere to strict guidelines
There are brand guidelines that are already built into an existing document. Even if nothing else is going to be the same, doing a Save As to keep the colors, fonts and logos alone is helpful.
If the content is going to follow a predictable structure
You know you need to use a specific deck template. Even if you're going to change all the content, this makes creating the new content easier because it becomes a bit fill-in-the-blank. Of course you'll add or delete parts, but I would guess around 30% is good to go with minor adjustments. This is also a good example of tricking yourself into starting.
If you're revising something
If you're taking something and adjusting copy or swapping out an image there's no need to start over.
Content First:
If you're doing something novel
You haven't done anything exactly like this before and/or it should be different than anything else.
If the form isn't pre-determined
What you're doing could be presented in multiple formats. Or, you need to figure out a new kind of format that works well for this content specifically.
If it helps you think things through
Sometimes you need to write an outline to get your thoughts together even if you know exactly what the format will be.
The only thing to look out for is to make sure you're naming all these copies of documents intentionally in a way that makes sense to you. Nobody wants to sift through your Final Pres copy copy copy copy copy copy.